Bicontinuous vitrimer heterogels with wide-span switchable stiffness-gated iontronic coordination

Currently, it remains challenging to balance intrinsic stiffness with programmability in most vitrimers. Simultaneously, coordinating materials with gel-like iontronic properties for intrinsic ion transmission while maintaining vitrimer programmable features remains underexplored. Here, we introduce a phase-engineering strategy to fabricate bicontinuous vitrimer heterogel (VHG) materials. Such VHGs exhibited high mechanical strength, with an elastic modulus of up to 116 MPa, a high strain performance exceeding 1000%, and a switchable stiffness ratio surpassing 5 × 103. Moreover, highly programmable reprocessing and shape memory morphing were realized owing to the ion liquid–enhanced VHG network reconfiguration. Derived from the ion transmission pathway in the ILgel, which responded to the wide-span switchable mechanics, the VHG iontronics had a unique bidirectional stiffness-gated piezoresistivity, coordinating both positive and negative piezoresistive properties. Our findings indicate that the VHG system can act as a foundational material in various promising applications, including smart sensors, soft machines, and bioelectronics.


and FTIR characterization SEM
Scanning electron microscopy (SEM) image of the PCL vitrimer framework was recorded on a SEM (HITACHI S-4800).The sample was prepared by utilizing the solvation effect of ethanol, undergoing three cycles of soaking for 12 hours and washing with ethanol to remove the ILgel phase of the VHG5.The sample was sputtered with a layer of Au using the magnetron sputtering instrument for SEM imaging.Moreover, the PCL vitrimer was prepared and used to characterize the infrared spectra by a Fourier-transform infrared (FTIR) spectrometer (Vertex 70, Bruker).

Mechanical measurements
The tensile and compressive tests were conducted using a tensile-compressive tester (Mark-10/ESM301).In the tests, dog bone-shaped samples (15 mm length × 5 mm width × 1 mm height) were subjected to in the tensile tests at a rate of 50 mm min −1 .The tensile strain (ε) was determined as the elongation (ΔL) divided by the initial length (L0) (ε = ΔL / L0 × 100%).The elastic modulus (E) was calculated from the initial linear region of stress-strain curve.Cylindershaped samples (5 mm diameter × 10 mm height) were used for compressive tests with a deformation rate of 10% of the sample height per minute.

Thermal analysis
A TGA analyzer, the Perkin-Elmer TGA 4000, was used to perform the thermogravimetric analysis (TGA) measurements.The VHG was heated at a rate of 5 °C min -1 with a dry nitrogen stream from room temperature to 500 °C.Differential scanning calorimetry (DSC) data were obtained using a TA Instrument (DSC Q2000) under dry nitrogen environment.The samples, sealed in aluminum pans, were scanned between 10 and 80°C at a scanning rate of 5 °C min -1 .

Rheological test
The rheological properties of the samples were investigated by a modular compact rheometer (Anton Paar, MCR 301).For frequency sweeps, a 15 mm parallel plate geometry was used with a measurement, a 30 mm parallel plate was used instead to improve the signal quality, as the samples were liquid-like.Measurements were taken at 80 °C, 15.8 rad/s, and a strain rate of 0.1%.Under a variety of temperatures, the storage modulus (G') of the samples was swept in a range of 0.1 ~ 100 rad s −1 or at 15.8 rad s −1 at a constant strain of 0.1 %.For creep measurements of the VHG5, a shear stress (10 Pa) was first applied for up to 45 min.Subsequently, the stress was removed, and the sample was allowed to recover for 45 min.

Stress relaxation
Stress relaxation tests were carried out using dynamic mechanical analysis (DMA Q800, TA instruments) in a "stress relaxation" mode with a strain value set at 10%.In stress relaxation tests, the samples were stretched under a constant strain of 10% in the temperatures ranging from 100 to 140 °C.The heating rate was 5 °C min -1 .

Swelling-deswelling tests
Weighted PCL-vitrimers (0.5 g) were soaked in 10 g of toluene for 24 h at room temperature until they reached reaching the swelling equilibrium.The samples were subsequently dried under vacuum for 24 h at 80 °C until reaching a constant value.During five cycles, the vitrimer contents were calculated as: 100% × final weight/ initial weight.

Shape memory and shape reconfiguration test
Shape fixity ratio (Rf) and shape recovery ratio (Rr) were computed as indicators of the shape memory property.The equations to calculate the parameters Rf and Rr are as follows: where ε, εf, and εr represent the strain under deformation at a temperature above Tm, the fixed strain in the temporary shape, and the original shape′s strain after shape recovering process, respectively.
The equations to calculate the parameters Rm and Rre are as follows: Rm or Rre = εp′ / εp ×100%, where εp and εp′ represent the processed strain induced by activating the bicontinuous network reconfiguration at 130 °C for 4 hours under an applied external force, and the sustained strain by reheating above Tm.

Piezoresistivity measurement
The piezoresistivity performance of the piezoresistive sensors was evaluated on a homemade test system consisting of a forcemeter (Mark-10/ESM301) and electrochemical workstation (CH Instruments, CHI660E).We tested the ion conductivity of both ILgel-iontronics and VHGiontronics with the temperature from -20 to 100 °C through the amperometric i-t test model.The piezoresistive sensor (4 × 4) arrays were constructed by encapsulating the VHG film between two poly (ethylene terephthalate) films coated with Ag electrodes.

Finite element modeling
Finite element modeling (COMSOL Multiphysics6.0,MA, USA) was employed to simulate the ionic transport pathway of the bicontinuous phase structure under deformation, the 3D model was built from 3DS MAX 2020(Autodesk, USA).In the simulation setup, Young's modulus of VFP at room temperature and high temperature were set as 208 MPa and 74 kPa, and Poisson ratios were 0.1 and 0.3 with the same density which was 1.1 g/ml.Young's modulus of IFP was 1.8 kPa at both room temperature and high temperature, that Poisson ratio was 0.3, and the density was 1.46 g/ml.The overall shape variable of the structure is set to 50%.Table S1.Comparison of the switchable stiffness ratio and strainmax for VHGs with previously reported typical switchable mechanics polymer materials, including vitrimers, shape memory polymers and gels, organohydrogels, stimuli-responsive gels, and ILgels.
Table S3.Shape reconfiguration ratio of the VHGs fabricated by various ILs containing [NTf2] as the anion.
Table S4.Various ion-liquids containing [Cnmim] as the cation were unable to produce a stable 284 bicontinuous VHG structure.
Table S5.Distinct from the unidirectional negative/positive features of any existing piezoresistive

Fig. S1 .
Fig. S1.The gelation process of the VHG.(A)The "one-step"orthogonal polymerization of the VHG material.(B) The gelation point of the VHG material.(C) The storage modulus (G') and loss modulus (G'') of the catalyst-free PCL vitrimer and the IL-PCL vitrimer during the gelation process.

Fig. S5 .
Fig. S5.The contents of the VFP network and the PCL vitrimer during the swelling-deswelling cycles.

Fig. S9 .
Fig. S9.Stable transitions between high and low elastic modulus of the VHG5 at 20°C and 80 °C.

Fig. S10 .
Fig. S10.The FTIR spectrum exhibited the characteristic absorption peaks of the residual

Fig. S11 .
Fig. S11.Stress relaxation tests.(A) Stress relaxation behaviors of the VHGs with different VFP and IFP components at 130 °C.(B) The related shear modulus of the VHG5.(C) Stress relaxation behaviors of the VHG5 as the temperature increased from 110 °C to 140 °C.

Fig. S12 .
Fig. S12.Stress relaxation behaviors of the catalyst-free PCL vitrimer, the VFP network, and the VHG7.5 at 130 °C (A), and the creep curve of the VHG5 at 130 °C under applied shear stress of 10 Pa (B).

Fig. S14 .
Fig. S14.TGA analysis of the original VHG (black line) and the VHG samples after stress

Fig. S19 .
Fig. S19.Sensing performance diversity of the VHG.(A) Optical photographs of the VHG7.5 with different microstructures (planar, micropillar, and microribbon).(B) Negative piezoresistive signal responses of the VHG7.5 iontronics with different microstructures and sensitivity to various forces at 20 °C.(C) Negative piezoresistive signal responses of the VHG7.5 iontronics with different microstructures under loading/unloading cycles at 20 °C.Scale bar, 1 mm.